On 7/31/01 1:08 PM, "Daniel J. Mitchell" <DanielMitchell@xxxxxxxxxxxxx>
wrote:
> Okay, so I'm obviously confused about something. I was assuming that the
> shorter the lens, the less glass there would be between the outside world
> and the film, so there would be less light reduction because the light is
> passing through a smaller amount of stuff. Though that, of course, is
> forgetting that faster lenses tend to be bigger at the front, which
> obviously lets more light in..
>
> But I'm still not sure why it is that the 50mm lens is the fastest -- is it
> just that there's not the demand for a $100,000 300mm f1.2 lens? Is it just
> the case that 50mm lenses happen to match up to 35mm film in a particularly
> handy way to allow making fast lenses at that size without it being too
> expensive?
>
> -- dan (perhaps I should just get a book on optics and this would all
> become clear..)
Anything longer than 50mm gets very large when you get a wider aperture.
Aperture is expressed as f/n is the size of the opening (where f is the
focal length, and n is the aperture number). That's why a 300mm 2.8 is so
huge. A 300mm 1.2 would be positively massive, and would also result in a
very large (and expensive) front element.
That answers telephoto side of things. The wide angle side of things is a
different. With a wide angle, there are difficulties an making a a large
curved lens for the front element. It is much easier (and cheaper) to make a
smaller front element because it's easier to keep that lens free of
distortion (or reasonably free). So for a wide angle, it's mainly a concern
with cost and the law of diminishing returns. The amount of effort (and
cost) required to get a faster wide angle lens becomes very great in order
to achieve a very small increase in speed.
As for why it's the 50mm, well that just has to do with what the 50 is. The
50mm is around the "normal" length for 35mm. In other words, it reproduces
an angle of view very close to our main angle of view (not including
peripheral vision). This happens to make it a very easy lens to design (I'm
not exactly sure why) yet it's also still fairly small. Because of this,
it's fairly cheap to design a fast lens at this length.
--
Andrew "Frugal" Dacey
frugal@xxxxxxxxxxxxxxx
http://www.tildefrugal.net/
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